As sizes of transistors decrease, a thin film such as an insulating film which constitutes a sidewall spacer (SWS) of a gate electrode or the like may require a low temperature for film forming, an improved resistance to hydrogen fluoride (HF), and a small dielectric constant. To this end, it has been studied to use a silicon borocarbonitride (SiBCN) film obtained by adding boron (B) and carbon (C) to a silicon nitride film (SiN film) as an insulating film.
Such an insulating film is often formed by an alternating supply method that alternately supplies several kinds of process gases because high step coverage characteristics are required. For example, using a silicon (Si)-containing gas as a precursor gas (i.e., a silicon precursor), borontrichloride (BCl3) gas or diborane (B2H6) gas as a boron precursor, ammonia (NH3) gas as a nitrogen source, and ethylene (C2H4) gas or propylene (C3H6) gas as a carbon source, a SiBCN film can be formed on a substrate by performing a predetermined number of times a cycle that sequentially supplies those process gases to the substrate. However, the method that separately supplies the silicon precursor, the boron precursor, the nitrogen source, and the carbon source leads to a longer time for performing a single cycle, which results in a low productivity of a film forming process. Furthermore, in the method as described above, it is difficult to increase a C concentration in the SiBCN film and thus improve the resistance to HF.